Optical gauges provide one of several techniques for measuring mechanical deformation. Their use is based on interference; they are precise but permit measuring only a reduced area of a piece and are difficult to apply in the field. The photoelasticity technique, which uses the principle of light diffraction, allows a gradient display (since it collects data on a larger surface) but it is of a qualitative type and cannot be easily adapted to automatic acquisition of data. The so-called piezoresistive deformation indicators or strain gauges are very well accepted in the market and provide accurate data promptly, but are difficult to manufacture and to apply. This is confirmed, e.g., by U.S. Pat. No. 3,626,256 which refers to a deformation-measuring device using (as an electrically-insulating material) a commercial resin which is highly resistant to heat; in order to deposit a layer of semiconducting material and to maintain it at high temperatures, it is necessary to use vacuum deposition or cathode sputtering in an argon atmosphere, both methods being well known in the art and, besides, complex and expensive. In European patent application EP-61404-A a stable resistive ink (cermet) is used for the same purpose. Said ink is printed directly on ceramic or insulating glass through a silk-screen printing method. The involved process has a disadvantage of requiring high temperatures.
Another method is that of GB patent 2141548-A wherein coal particles (as an electric conducting material) are dispersed throughout a polymer (as a cementing agent) and with the addition of conducting terminals. This method, however, requires anodizing a surface to which a layer of the prepared material is applied.